It is well known that all biology electrical signal testing are carried out in a circumstance of strong background interference or existing patient pillaring voltage. In this connection, an amplifier has to be used because of strong interference (power-frequency interference especially). Unfortunately, biology electrical signals are usually weak and have to be amplified hundreds times or more. In addition, polarizing voltage is generated when electrode touches skin, which makes the first gain of amplifier weak. Then a second amplifying has to be carried out after isolating polarizing voltage by resistance-capacitance circuits. However, because of time constant circuits, the capacitors shall be charged when patients' polarizing voltage is high which makes the first output saturated. As if the patient is in a stable state (the maximum polarizing voltage reaches a low normal value), then it needs a very long time to release the charge of stop capacitor, which makes it is impossible to gather ECG signal during this period. So the communication device is slow in generating signals and its base line is easy to be shifted but slow to be recovered. To sum up, the traditional AC magnitude has the following defects such as small signal dynamic range, complex circuits, big noise, saturated amplifier, slow baseline recovery (baseline shifting), pacemaker (PACE) pulse testing, weak noise immune, losing signal direct current or AC signals similar to direct current signals and so on.
To solve the above stated problems, direct current amplifier circuits with instrumentation amplifier is popularly applied, however, the traditional direct current amplifier still have the following defects;
Firstly, the circuits is still complicated and with many amplifying steps, which is not good for controlling system noises and weak in restraining common mode interference. At the same time, all the signals from instrumentation amplifier are all single-ended signals, which makes it is impossible to eliminate the common mode interference caused by signals joined incidentally.
Secondly, it is costly. Almost all the instrumentation amplifier is with a high price. Even if the instrumentation amplifier put up by trinary operational amplifier is adopted, on the one hand the cost is still much higher than the non-inverting parallel amplifier; on the other hand its common mode rejection ratio is hard to be enhanced because of low precision of resistance matching. Usually, 60 dB is already very good.
In conclusion, to adopt traditional direct current amplifier circuits is hard to solve the problems of high cost and low common mode rejection ratio.